Skin biopsies from the mother of a classical case of the Lesch-Nyhan syndrome grew only wild type fibroblasts. This suggested that she is not a heterozygous carrier of the mutant X-linked structural gene which causes the syndrome, and that a fresh mutation caused the disorder in her son. Evidence of mosaicism was sought in skin fibroblasts, hair follicles, jejunal mucosa, cultured bone marrow cells and phytohaemagglutinin (PHA) stimulated lymphocytes from known hemizygotes and heterozygotes for the so-called complete, and partial deficiencies of hypoxanthine guanine phosphoribosyltransferase (HGPRT). These studies were designed to determine the genetic status of the mother of the propositus and to determine if the genetic diagnosis could be improved by the simultaneous study of this wider range of tissues. The results are compatible with the mother of the propositus being a non carrier of a mutant gene causing the Lesch-Nyhan syndrome in her son. Only the study of cultured skin fibroblasts, and of the enzyme levels in hair follicles, contributed diagnostically useful information in this case.